GEOCHEMISTRY OF LATE PERMIAN NONMARINE BIVALVES - IMPLICATIONS FOR THE CONTINENTAL PALEOHYDROLOGY AND PALEOCLIMATOLOGY OF NORTHWESTERN CHINA

The Upper Permian Wutonggou Formation, near Turpan, Xinjiang Autonomou s Region, People's Republic of China, consists of about 60 m of siltst ones, sandstones and limestones. A 15 cm thick limestone bed about 40 m above the base, contains abundant bivalve shells of the species Pala eanodonta fisheri. P. castor and Palaeomutela sp. These relatively thi ck shells are chalky-white and the internal microstructure is preserve d in some specimens as complex-crossed lamellas, which are always arag onite, and prismatic fibers which may be aragonite or calcite. Mineral ogical tests confirm aragonite in microstructurally pristine specimens . This is the oldest-known locality for biogenic aragonite in China. T he bivalves of northwestern China are geochemically distinct from typi cally marine and terrestrial counterparts. Their Sr, Mg, Mn, and Fe co ntents are strongly habitat controlled by ambient environmental condit ions and support the postulated lacustrine paleoecology for the limest ones of the Wutonggou Formation. Modelling of deltaC-13 values (-2.9 t o -3.9 parts per thousand, PDB) and Fe suggests that Palaeanodonta and Palaeomutela preferred shallow waters and the values are well within the seasonal variation expected for north temperate lakes. The light a nd relatively constant deltaO-18 values (-12.0 to -12.1 parts per thou sand, PDB) of the bivalves suggest that Wutonggou surface lake waters were probably well mixed. The modeled deltaO-18 value of -11 to -15 pa rts per thousand (SMOW) for annual meteoric precipitation, influenced by snow meltwater, appears reasonable for situated about 50-60-degrees -N of the Permian equator.